Microwave bulk-effect negative-resistance device having a high perimeter to area ratio



34 v v 4 33 INVENTOR. X I CHARLESHMQSHE-R Sept. 23, 1969 c. H. MOSHER3,469,209

MICROWAVE BULK-EFFECT NEGATIVE RESISTANCE DEVICE HAVING A HIGH PERIMETERTO AREA RATTO Filed June 50, 1967 FIG. I

PRIOR ART ACTIVE REGION F ILL FIG.II

United States Patent US. Cl. 331-107 Claims ABSTRACT OF THE DISCLOSURE Amicrowave bulk-effect negative-resistance semicond uctive device isdisclosed. The bulk-effect semiconductive device is formed and arrangedas a ring of semiconductive material or as a ring-shaped array ofsemiconductwo segments. The ring-shape serves to remove semiconductivematerial from the central region of the device where the microwavecurrent density is low due to skin effect. In this manner, the currentdensity is nearly constant across the device to limit transverse domainpropagation which can cause burnout. The radial thickness of the ring orsegments of the ring-shaped device is preferably less than a skin depthat the operating frequency of the device and the circumference ispreferably less than a half wavelength to prevent unwanted modes ofoscillation. The ring-shape increases the power handling capa'bilitiesof transit-time-mode Gunn-effect devices and improves the efficiency ofGunn devices operated in the limited space charge accumulation mode.

DESCRIPTION OF THE PRIOR ART Heretofore, two bulk-effectnegative-resistance semiconductive devices have been operated inparallel to obtain higher power output at L-band. Such devices aredescribed in an article titled, High-Peak-Power Gallium ArsenideOscillators, appearing in the IEEE Transactions on Electron Devices,vol. ED-l3, No. 1, pages 105-110, January 1966. These devices provided205 watts peak power at 1540 mHz. The devices were each about 1 squaremillimeter in cross-sectional area, about 100 microns thick, and had aresistance of 0.89. It turns out that the devices cannot be operatedwithout failure above a certain voltage which is about three times thethreshold bias voltage, -i.e., the bias voltage at which microwaveoscillations will start. Also, the DC current for a given device remainsapproximately constant with increased bias voltage above thresholdvoltage. Thus, the obvious way to increase the output power is todecrease the resistance of the devices such that, at their operatingvoltage, they conduct more DC current. Given a certain resistivity andthickness of the semiconductive material, the current is increased byincreasing the cross-sectional area of the semiconductive device. Whenthe cross-sectional area of the semiconductive device is increased abovea certain value, as of 1-2 square millimeters at L-band to provide a lowfield DC resistance of less than 0.59, it was found that the deviceswould burn out.

SUMMARY OF THE PRESENT INVENTION The principal object of the presentinvention is the provision of an improved bulk-effectnegative-resistance microwave semiconductive device.

One feature of the present invention is the provision of a bulk-effectnegative-resistance semiconductive device wherein the activesemiconductive material is hollow in the central region and confined toa region near the perimeter of the device as defined by the highestdensity microwave magnetic field lines circumscribing the device inoperation whereby unwanted burn-outs of the semi- ICC conductive deviceare minimized while providing low field device resistances less than0.59 for high power operation.

Another feature of the present invention is the same as the precedingfeature wherein the radial width of the semiconductive material is lessthan 1.5 skin depths.

Another feature of the present invention is the same as any one or moreof the preceding features wherein the semiconductive device is formed byan array of smaller semiconductive elements disposed in the region nearthe perimeter of the composite device, whereby each element of the arraycan be individually selected and tested for improving the probability ofhaving a composite device which is free of semiconductive defects.

Another feature of the present invention is the same as the precedingfeature wherein the individual semiconductive elements are shaped in theform of a segment of a ring with rounded corners, whereby a compositeringshaped array of elements is obtained without sharp corners whichwould otherwise tend ot promote burnout of the device at high powerlevels.

Another feature of the present invention is the same as any one or moreof the preceding features wherein the semiconductive material has across sectional area less than one half of the area bounded by theperimeter defined by the highest density of enclosing microwave magneticfield lines.

Other features and advantages of the present invention will becomeapparent upon a perusal of the following specification taken inconnection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plot of DC current Iversus DC bias voltage V for a bulk-effect negative-resistancesemiconductive device incorporating features of the present invention,

FIG. 2 is a perspective view of a semiconductive wafer of the prior artand depicting the burnout region,

FIG. 3 is a plot of instantaneous current density J, as a high electricfield domain is forming versus radius for the structure of FIG. 2,

FIG. 4 is a perspective view of a semiconductive device incorporatingfeatures of the present invention,

FIG. 5 is a plan view of an alternative device of the present invention,

FIGS. 6 and 7 are plan views of alternative devices of the presentinvention,

FIG. 8 is a longitudinal sectional view of a composite semiconductivedevice of the present invention,

FIG. 9 is a sectional view of the structure of FIG. 8 taken along line9-9 in the direction of the arrows,

FIG. 10 is a schematic circuit diagram of a high power microwaveoscillator employing a device of the present invention, and

FIG. 11 is a sectional view of a device similar to that of FIG. 4depicting an alternative embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, thereis shown a typical plot of DC current I versus DC bias voltage V asapplied across terminals of a bulk-effect negative-resistancesemiconductive device. It is found that with increased bias voltage theDC current increases until a certain threshold value of voltage V isreached. At the threshold value of voltage, the device will interactwith microwave fields of a circuit to produce microwave oscillations andto convert DC power into microwave power. Such bulk-effect devicesinclude Gunn devices operable in any one of a number of modes. Thebulk-effect conversion process is due predominately to the properties ofthe bulk semiconductive material as contrasted with other types ofnegative-resistance devices which rely primarily upon the properties ofa p-n junction for the power conversion process. Typical of suchjunction devices is the tunnel diode. The advantage of bulk-effectnegative-resistance devices is that they are capable of operating tomuch higher power levels because the power is dissipated in the threedimensional bulk of the semiconductive material as compared with powerdissipation in a thin p-n junction region.

Above the threshold voltage V the DC current remains nearly constantwith increasing voltage V. Thus, the power output may be increased byincreasing the bias voltage. However, the bias voltage should not beincreased above about 3V as this will shorten operating life. This,then, sets the upper limit on power output for a given device and it isseen that the power output can then only be increased by increasing theDC current which can be done by lowering the DC low field resistance,i.e., resistance below threshold V For a semiconductive material havinga given resistivity and thickness, the resistance can only be decreasedby increasing the cross-sectional area of the device. However, it wasdiscovered that when the resistance was lowered below 0.582 that thedevices failed by burnouts which occurred most frequently in a ring-likeregion 1 of the device 2, as shown in FIG. 2.

It is believed that the bumouts occur in this region because theinstantaneous current density J is not uniform across the bulk material.More particularly, due to skin effect, when the device 2 has a diameterof about 2 skin depths or more, as shown in FIG. 3, the current densityI will be less near the central region of the device 2 as compared withthe current density J at the perimeter of the device. The disturbancewhich will become the high field domain will tend to propagatetransversely toward the central region of the device as indicated by thepeaked inwardly directed wavefronts A sketched on the diagram of FIG. 3.The result is that excessive electric fields and current densities areproduced resulting in a breakdown of the material with a resultantburnout. The adverse effects of transverse propagation of disturbanceshave been observed for the operation in the various domain modes. It hasnot been observed, to date, in the (LSA) mode of operation.

Referring now to FIG. 4, there is shown a bulk-effectnegative-resistance semiconductive device of the present invention. Thedevice 3 is essentially the same as the prior art large area Gunn effectdevices except the central region of bulk semiconductive member has beenremoved to place the active area of the device near its perimeter ascontrasted with a solid cross-sectional area device 2, as shown in FIG.2. The area near the perimeter can be made sufiiciently large to producea low resistance device 3, i.e., resistance less than 0.59. The devicewill have an effectively uniform current density J across the ring ofsemiconductive material 4 since its radial thickness is selected to beless than 1.5 skin depths.

The parameter of interest is that the semiconductive material should beconfined to an area of the device near its perimeter as defined by theloop of most intense microwave magnetic field circumscribing the device.For a circular disk of semiconductive material, the loop of most intensemicrowave magnetic field which circumscribes the semiconductivestructure coincides with the perimeter of the disk. In other geometries,the loop defined perimeter is more difficult to determine. To obtain asubstantial improvement over a solid disk of semiconductive material, atleast half of the cross-sectional area bounded by the loop definedperimeter of the semiconductive structure should be removed. Thus, for acircular cross-section semiconductive structure a ring, as shown in FIG.4, offers the optimum geometry. The ratio of areas bounded to thatremaining at the perimeter can be reduced to the expression:

where P is the perimeter of the disk, and A is the area occupied by thering of semiconductive material 4. If the disk were solid, as in FIG. 2,the ratio of Eq. 1 would be unity. Thus, for a circular ring-shapedsemiconductive structure 4 of the present invention the ratio of Eq. 1should be at least 2 and preferably as high as possible consistent witha preferred operating condition that the perimeter of the structure beless than one half an electrical wavelength long at the operatingfrequency in the semiconductive material in order to avoid possiblecircumferential modes of oscillation in the device 3.

A suitable semiconductive material for the ring 4 includes n-type GaAshaving no dislocations or deep donors in the crystal lattice and havinga positive temperature coelficient of resistivity.

A pair of annular electrodes 5 and 6, as of nickel-tin, are alloyed tothe ends of the semiconductive material 4 in the manner as described inan article titled, Microwave Phenomena in Bulk GaAs, IEEE Transactionson Electron Devices, vol. ED-13, No. 1, at pages 94-105, January 1966.

The axial thickness of the semiconductive body 4 is determined by itsintended mode of operation. In a transit time mode, this thickness isabout 3-4 mils at L-band and about 0.5 mil at X-band.

Referring now to FIG. 5, there is shown, in plan view, an alternativerectangular ring-shaped semiconductive device 8 wherein the area of thesemiconductive structure is less than one half the area bounded by theperimeter defined by the loop of highest intensity microwave magneticfield H. Roughly, the highest intensity magnetic field is at theperimeter of the device 8. The central area, which is removed, is atleast equal to the remaining area of the ring and preferably muchgreater than the area of the ring.

Referring now to FIGS. 6 and 7, there are shown alternative embodimentsof the present invention. In these embodiments, the semicondutivedevices 13 and 14, respectively, are made up of a number of segments 15of bulk-effect negative-resistance semiconductive material arranged inand confined to a region near the perimeter defined by the most intenseclosed loop of microwave magnetic field H.

The segments 15 are preferably rounded on the corners to preventconcentration of circumferential magnetic fields and, thus, axialmicrowave currents. The individual segments 15 each include their ownelectrodes for applying DC and microwave potentials. The individualsegments 15 are connected in parallel to form the composite devices 13and 14, respectively.

One advantage of the segmented geometries of FIGS. 6 and 7 is that theindividual semiconductive segments 15 can be separately tested andselected for proper operating characteristics. In the single ring-shapedsemiconductive elements of FIGS. 4 and 5, it is often difficult to growa single perfect crystal of sufficient size to permit cutting out therelatively large rings of FIGS. 4 and 5. On the other hand, relativelysmall segments 15 of perfect material are more easily produced.Moreover, with the segmented ring-shapes of FIGS. 6 and 7, transversecurrent domain propagation, as indicated in FIG. 3, is impeded by thebreaks in the semiconductive material.

Also, as indicated in the embodiment of FIG. 7, the segments 15 need notform a complete circle but need only form a generally C-shaped array orstructure. Such a C-shaped semiconductive structure will define asufficiently large hollow interior region. The intense loop of microwavemagnetic field as indicated by the dotted line H of FIG. 7, should notdip too far into the central region of the structure as this willsubstantially decrease the area enclosed by the intense magnetic fieldand reduce the ratio of bounded area to actual area of the device.

Referring now to FIGS. 8 and 9, there is shown a packaged bulk-effectnegative-resistance device made up of a plurality of individual segments15 arranged in a circular array near the perimeter defined by the closedloop of most intense microwave magnetic field. The segments 15 are eachabout 3 to 4 mils thick and 40 mils on a side (40 mils square) and thereare about 12 segments 15 in the array to form an L-band device 17. Eachsegment 15 has a pair of electrodes on opposite sides. The electrodes onone side are soldered to the fiat end of a stud 18 which is made of agood electrical and thermally conductive material such as Te-Cu to serveas one microwave terminal and as a heat sink.

An array of conductive tabs 19, as of gold, 40 mils in width and 1 milthick and 80 mils in length interconnect the upper electrodes of thesegments 15 with the metallized end of a ceramic cylinder 21. Thecylinder 21 is brazed to a shoulder of the stud 18 and surrounds thearray of segments 15. A kovar cap 22 is soldered at its lip 23 to theouter ends of the tabs 19 to form the other terminal of the device 17.The stud 18 is provided with external threads to permit it to be screwedinto a microwave circuit.

Briefly, the circuit of FIG. comprises a pair of half wave opencircuited resonant sections. of transmission lines 24 and 25 containedin a conductive housing 26. Tuning capacitors 27 are provided at theopen circuited ends of the transmission lines 24 and 25 for tuning theirresonant frequencies and for shifting the position of their microwavevoltage nulls which typically occur centrally of their lengths.

The bulk-effect negative-resistance device 17 of FIGS. 8 and 9 isscrewed through the housing 26 for connection across the resonant line24 at a point near the voltage null point to provide a low impedancematch to the device 17. An output coaxial line 31 has its centerconductor 32 connected to the output resonant line 25 near its midpointfor impedance matching. A low impedance source of bias voltage 33, whichmay be pulsed is connected to the microwave voltage null point of theinput resonant line 24 for reducing microwave coupling to the source 33and for applying operating bias voltage across the bulk-effect device 17A conductive septum 34 extends partially across the housing 26 to definean inductive coupling iris 35 between the two resonant lines 24 and 25for controlling the degree of coupling therebetween.

In operation, the microwave fields of the input resonator 24 interactwith the charge carriers in the bulk-effect device 17 to produce amicrowave output signal that is coupled to a load via output coaxialline 31. A peak power output of 500 watts at 1 gHz. has been produced bya device 17 having about a dozen segments arranged as shown in FIGS. 8and 9 and operated in a Gunn-effect domain mode. A similar device 17having the segments 15 dimensioned for operation in the limited spacecharge accumulation (LSA) mode should provide higher output power withincreased efficiency as compared with operation in the LSA mode with asolid non-ring shaped device. Operation on the (LSA) mode is describedin an article titled, A New Mode of Operation for Bulk NegativeResistance Oscillators, John A. Copeland, Proc. IEEE, vol. 54, #10, pp.14791480, October 1966. The ring-shaped LSA mode device may not haveadvantages with respect to device failure but should provide increasedefficiency due to skin depth consideration.

The gist of the present invention is to confine the active region of thebulk-effect negative-resistance device to a region near its perimeter.As thus far described, this has been accomplished, in practice, byremoving the semiconductive material from all regions but the perimeter.As an alternative, where the dimensions of the device do not permitremoval of the central region, one or both of the electrodes 5 and 6 forapplying the bias and microwave potentials may be removed from thecentral region, as shown in FIG. 11. Since the active region of thesemiconductive device 4 is confined to regions which experience a biasvoltage in excess of V the central region of the semiconductive material4 will not be active.

Since many changes could be made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A bulk-effect negative-resistance semiconductive microwave deviceincluding, means forming semiconductive structure having an activeregion exhibiting bulk-effect negative-resistance with the applicationof a certain bias voltage across said semiconductive structure, meansforming a pair of electrode structures affixed to opposite sides of saidsemiconductive structure for applying the certain bias voltage and forproviding a pair of microwave equipotential surfaces on opposite sidesof said semiconductive structure to support a microwave potential and abias potential thereacross for electromagnetic interaction with chargecarriers in said bulk-eflFect structure, the improvement wherein, saidsemiconductive structure is formed and arranged such that the activesemiconductive material is confined to a region of the structure whichis near the perimeter thereof, and wherein the central region of saidstructure is free of said active semiconductive material.

2. The apparatus of claim 1 wherein said region located near theperimeter of said semiconductive structure and within which said activesemiconductive material is confined has a radial thickness less than 1.5skin depths for the frequency of the microwave potential to beinteracted with the device.

3. The apparatus of claim 1 wherein said semiconductive structureincludes a plurality of segments of active semiconductive materialdisposed in said region near the perimeter of said structure.

4. The apparatus of claim 1 wherein said semiconductive device has a lowfield DC resistance of less than 0.59.

5. The apparatus of claim 1 wherein said active semiconductive materialis confined to a ring-shaped region of space.

6. The apparatus of claim 1 wherein said semiconductive structure is asegmented ring-shaped structure of semiconductive material.

7. The apparatus of claim 1 wherein the microwave current flowingthrough said semiconductive structure in operation produces a microwavemagnetic field to fix a perimeter defined by the most intense closedloop of microwave magnetic field which encircles the device and whereinthe area bounded by the perimeter defined by the microwave magneticfield is more than twice the area occupied by said active semiconductivestructure.

8. The apparatus of claim 1 including an electrically conductive stud,and wherein one of said electrode structures which is affixed to saidactive semiconductive structure is bonded to the end of said stud formaking an electrical connection thereto and for heat sinking thesemiconductive device.

9. The apparatus of claim 8 wherein said semiconductive structureincludes a circular array of semiconductive segments.

10. The apparatus of claim 8 wherein the perimeter of saidsemiconductive structure is less than one-half an electrical wavelengthlong at the operating microwave frequency of the bulk-effect device.

No references cited.

JOHN KOMINSKI, Primary Examiner U.S. Cl. X.R.

